[PATCH v3 3/7] libfs: Validate negative dentries in case-insensitive directories

[Gabriel Krisman Bertazi <krisman@suse.de>]

From: Gabriel Krisman Bertazi <krisman@collabora.com>

Introduce a dentry revalidation helper to be used by case-insensitive
filesystems to check if it is safe to reuse a negative dentry.

A negative dentry is safe to be reused on a case-insensitive lookup if
it was created during a case-insensitive lookup and this is not a lookup
that will instantiate a dentry. If this is a creation lookup, we also
need to make sure the name matches sensitively the name under lookup in
order to assure the name preserving semantics.

dentry->d_name is only checked by the case-insensitive d_revalidate hook
in the LOOKUP_CREATE/LOOKUP_RENAME_TARGET case since, for these cases,
d_revalidate is always called with the parent inode locked, and
therefore the name cannot change from under us.

d_revalidate is only called in 4 places: lookup_dcache, __lookup_slow,
lookup_open and lookup_fast:

  - lookup_dcache always calls it with zeroed flags, with the exception
  of when coming from __lookup_hash, which needs the parent locked
  already, for instance in the open/creation path, which is locked in
  open_last_lookups.

  - In __lookup_slow, either the parent inode is locked by the
  caller (lookup_slow), or it is called with no
  flags (lookup_one/lookup_one_len).

  - lookup_open also requires the parent to be locked in the creation
  case, which is done in open_last_lookups.

  - lookup_fast will indeed be called with the parent unlocked, but it
  shouldn't be called with LOOKUP_CREATE.  Either it is called in the
  link_path_walk, where nd->flags doesn't have LOOKUP_CREATE yet or in
  open_last_lookups. But, in this case, it also never has LOOKUP_CREATE,
  because it is only called on the !O_CREAT case, which means op->intent
  doesn't have LOOKUP_CREAT (set in build_open_flags only if O_CREAT is
  set).

Finally, for the LOOKUP_RENAME_TARGET, we are doing a rename, so the
parents inodes are also be locked.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>

---
Changes since v2:
  - Add comments to all rejection cases (eric)
  - safeguard against filesystem creating dentries without LOOKUP flags
---
 fs/libfs.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/fs/libfs.c b/fs/libfs.c
index 5b851315eeed..dd213f446427 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1462,9 +1462,57 @@ static int generic_ci_d_hash(const struct dentry *dentry, struct qstr *str)
 	return 0;
 }
 
+static inline int generic_ci_d_revalidate(struct dentry *dentry,
+					  const struct qstr *name,
+					  unsigned int flags)
+{
+	if (d_is_negative(dentry)) {
+		const struct dentry *parent = READ_ONCE(dentry->d_parent);
+		const struct inode *dir = READ_ONCE(parent->d_inode);
+
+		if (dir && needs_casefold(dir)) {
+			/*
+			 * Filesystems will call into d_revalidate without
+			 * setting LOOKUP_ flags even for file creation(see
+			 * lookup_one* variants).  Reject negative dentries in
+			 * this case, since we can't know for sure it won't be
+			 * used for creation.
+			 */
+			if (!flags)
+				return 0;
+
+			/*
+			 * Negative dentries created prior to turning the
+			 * directory case-insensitive cannot be trusted, since
+			 * they don't ensure any possible case version of the
+			 * filename doesn't exist.
+			 */
+			if (!d_is_casefold_lookup(dentry))
+				return 0;
+
+			if (flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET)) {
+				/*
+				 * ->d_name won't change from under us in the
+				 * creation path only, since d_revalidate during
+				 * creation and renames is always called with
+				 * the parent inode locked.  It isn't the case
+				 * for all lookup callpaths, so ->d_name must
+				 * not be touched outside
+				 * (LOOKUP_CREATE|LOOKUP_RENAME_TARGET) context.
+				 */
+				if (dentry->d_name.len != name->len ||
+				    memcmp(dentry->d_name.name, name->name, name->len))
+					return 0;
+			}
+		}
+	}
+	return 1;
+}
+
 static const struct dentry_operations generic_ci_dentry_ops = {
 	.d_hash = generic_ci_d_hash,
 	.d_compare = generic_ci_d_compare,
+	.d_revalidate_name = generic_ci_d_revalidate,
 };
 #endif
 
-- 
2.41.0